Crack Paths 2012

Microstructural investigations of crack path development in

nodular cast iron under variable amplitude loading

L. Zybell1*, H. Chaves1, M. Kuna1,

T. Mottitschka2, S. Henkel2, G. Pusch2, H. Biermann2

1 T UBergakademie Freiberg, Institute of Mechanics and Fluid Dynamics

Lampadiusstr. 4, 09596 Freiberg, Germany

2 T U Bergakademie Freiberg, Institute of Materials Engineering

Gustav-Zeuner-Str. 5, 09596 Freiberg, Germany

* lutz.zybell@imfd.tu-freiberg.de

In this paper new results of optical in-situ investigations of overload effects during

fatigue crack growth in nodular cast iron are presented. Systematic variable amplitude

loading experiments have been performed on SENB3 specimens and cruciform

specimens under biaxial loading in order to study the influence of the overload ratio on

the microstructural crack growth. With the obtained high-quality images of the crack

initiation process the overload effects, showing crack branching, micro crack initiation

in front of the main crack tip and micro crack coalescence, are identified as early

stages of ductile fracture initiation. Furthermore, the results of the optical insitu

investigations are compared to crack length measurements. Finally, the experimental

findings are discussed with respect to results of recent finite element studies of the

ductile failure initiation in nodular cast iron.

I N T R O D U C T I O N

Due to its good mechanical and fracture mechanical properties nodular cast iron is today

widely used in engineering applications. Because of the inhomogeneity of the material,

consisting of a ferritic matrix with embedded nodular graphite particles, the probability

of fatigue cracks emanating from those particles must be taken into account within the

fracture mechanics assessment of structures made of nodular cast iron. Therefore, the

fracture mechanical behaviour of the nodular cast iron EN-GJS-400 18LT has been

investigated experimentally and numerically within this study.

Fatigue crack growth in nodular cast iron under constant amplitude loading was

extensively studied in the past. Regarding variable amplitude loading, Hübner et al. [1]

detected considerable crack growth acceleration when overloads were applied, what is

in contrast to homogeneous materials like steel and aluminium [2]. Further

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